The present invention relates to a fluid reservoir for an hydraulic brake master cylinder, and more particularly to an improved sealed fluid reservoir in which brake fluid is constantly sealed against the surrounding air regardless of increase or decrease in the amount of brake fluid.
In conventional sealed reservoirs of this kind, an extendable diaphragm seal member is assembled with the opening of the reservoir casing to provide a fluid-tight seal against the surrounding air for the brake fluid stored within the casing and to prevent entrance of water into the brake fluid. In being adapted to the diaphragm seal member, the size of the fluid reservoir must be made larger because the interior of the casing is occupied by the diaphragm seal member. Furthermore, if the diaphragm is designed as a thin film to minimize the volume of the seal member so as to permit the reservoir to be made smaller, the diaphragm seal member becomes less durable and higher in production cost because of difficulty of fabrication.